The present invention relates generally to electronic circuits and, more particularly, to a method and apparatus to generate an optimum common-mode voltage in analog differential circuits.
Generally, the dynamic range of analog circuits is limited by the maximum output voltage swing and the electronic noise within those circuits. The electronic noise may be minimized by increasing the power dissipation and/or the silicon area of the circuits. However, it may be also desirable to extend the dynamic range of the analog circuitry in integrated circuits by maximizing the available linear output voltage swing.
One known solution to increase the linear output voltage; swing involves the use of differential architectures, which double the output voltage swing of a typical single-ended implementation. However, the resulting output voltage swing is still limited to a fraction of the power supply voltage.
The information content in typical differential circuits is carried by the difference in value of two output signals moving in opposite directions. The common-mode level of the output signals within such typical differential circuits needs to be set to a specific value and all such differential circuits use one of many known mechanisms to adjust the average of the two output signals at the common-mode level. However, in all such differential circuits, the common-mode level is either fixed, e.g., being set by a band gap voltage, or it tracks the power voltage supply, e.g., being set at mid-supply.
A method and apparatus to generate an optimum common-mode voltage in analog differential circuits are described. A first output voltage is generated as a function of a power supply voltage and a positive saturation voltage in a differential amplifier circuit. A second output voltage is then generated as a function of a negative saturation voltage in the differential amplifier circuit. An optimum common-mode level output voltage is then calculated as an average of said first output voltage and said second output voltage to obtain a linear output range of said differential amplifier circuit.